1. Field of the Invention
The present invention relates to an electroluminescent element (suitable for the flat display of self-luminous type which employs an organic thin film as the luminescent layer) and a luminescent display employing the same.
2. Prior Art
Nowadays, growing importance is attached to the human interface with machines, particularly multimedia-oriented ones. For efficient, comfortable operation, man needs ample, correct, concise information from the machine being operated. To this end, much has been studied about display elements.
Among other things, the lightweight, high-efficiency flat panel display is expected to be suitable as the computer screen and television screen. On the other hand, at present the cathode-ray tube is most commonly used as display because of its high luminance and good color reproducibility. However, it still has the disadvantage of being bulky and heavy and consuming much electricity, which should be eliminated in the future.
An example of the flat panel display is the liquid-crystal display of active matrix type which is commercially available. Unfortunately, it suffers several disadvantages. That is, it has a narrow viewing angle; it consumes much electricity for back-light during use in the dark (because it is not self-luminous); it does not respond quickly to the high-speed fine video signals which are expected to be put to practical use in the future; and it costs money for the large-sized screen.
A potential substitute for this is the light-emitting diode. However, it still has a problem with production cost. In addition, it involves difficulty in producing a matrix of light-emitting diodes on a single substrate. There still is a long distance to go for the practical, low-priced substitute for the cathode-ray tube.
A promising flat panel display free from the above-mentioned disadvantages is one which employs an organic luminescent material. By virtue of organic luminescent material, this flat panel display is self-luminous, capable of high-speed response, and independent of viewing angle.
FIG. 1 shows an example of the conventional electroluminescent (EL) element 10 that employs an organic luminescent material. This organic EL element 10 is of double-hetero type which is composed of an ITO (indium tin oxide) transparent electrode 5, a hole transport layer 4, a luminescent layer 3, an electron transport layer 2, and an cathode (such as aluminum electrode) 1, which are formed consecutively by vacuum deposition on a transparent substrate (such as glass substrate) 6.
It functions when a dc voltage 7 is selectively applied across the transparent electrode 5 (as the anode) and the cathode 1. This voltage application injects holes (as carriers) from the transparent electrode 5 and injects electrons from the cathode 1. The holes move through the hole transport layer 4 and the electrons move though the electron transfer layer 2. Thus the electron-hole recombination occurs, thereby emitting the light 8 of prescribed wavelength which is visible through the transparent substrate 6.
The luminescent layer 3 contains a luminescent substance such as anthracene, naphthalene, phenanthrene, pyrene, curine, perylene, butadiene, coumarin, acridine, stilbene, and europium complex. The luminescent substance may be contained in the electron transport layer 2.
FIG. 2 shows another example of the conventional electroluminescent (EL) element 20 that employs an organic luminescent material. This organic EL element 20 is of single-hetero type which lacks the luminescent layer 3. Instead, it has the luminescent substance contained in the electron transport layer 2, so that light 18 of prescribed wavelength emits from the interface between the electron transport layer 2 and the hole transport layer 4.
FIG. 3 shows an example of the above-mentioned organic EL element in practical use. It has the organic layers (the hole transport layer 4 and the electron transport layer 2 or the luminescent layer 3) in the form of stripy laminates. The stripy laminates are interposed between stripy cathodes 1 and stripy anodes 5 which intersect with each other so that these electrodes form a matrix. The matrix receives signal voltages sequentially from the luminance signal circuit 40 and the control circuit 41 containing a shift register, so that a number of intersections (pixels) emit light.
This construction permits the EL element to be used as a display unit as well as an image reproducing unit. Incidentally, the EL element produces full-color or multi-color if each stripy pattern is assigned to R (red), G (green), and B (blue).
In the above-mentioned display device composed of a plurality of pixels, the organic EL element is constructed such that the luminescent organic thin film layers 2, 3, and 4 are held between the transparent electrode 5 and the metal electrode 1 so that light is visible through the transparent electrode 5.
As mentioned above, an organic thin film containing a luminescent material is held between a transparent anode and a metal cathode so as to construct the organic luminescent element.
An organic electroluminescent element of double-hetero structure in which a perylene thin film is used for red luminescence was reported by C. Adachi, S. Tokito, T. Tsutui, and S. Saito in Japanese Journal of Applied Physics, vol. 27, No. 2, pp. L269-L271 (1988). There was reported an investigation into the use of a perylene derivative as the luminescent material.
For example, the use of a thin film of N,N'-dimethyl-3,4,9,10-perylenedicarboimide held between metal electrodes was reported by M. Hiramoto, T. Imahigashi, and M. Yokoyamo in Applied Physics Letters, vol. 64, No. 2, pp. 187-189.
An organic EL element of double-hetero structure which employs as the luminescent layer a thin film of N,N'-bis (2,5-di-tert-butylphenyl)-3,4,9,10-perylenedicarboimide held between the hole transport layer and the electron transport layer was also reported by T. Katsunuma, M. Hiramoto, and M. Yokoyama in Applied Physics Letters, vol. 64, No. 19, pp. 2546-2548.
An organic EL element that emits red light by means of a tris (theonyltrifluoroacetonato) -Eu (III) complex contained in poly(methylphenylsilane) was reported by J. Kido, K. Nagai, Y. Okamoto, and T. Skotheim in Chemistry Letters, 1991, pp. 1267-1270.
An organic EL element of double-hetero structure which employs as the luminescent layer a tris(1,3-diphenyl-1,3-propanedino) (1,10-phenanthroline)-Eu (III) complex contained in 2-(4-biphenyl)-5-phenyl-1,3,4-oxazol was reported by J. Kido, H. Hayase, K. Hongawa, K. Nagai, and K. Okuyama in Applied Physics Letters, vol. 65, No. 17, pp. 2124-2126 (1994).
An organic EL element of double-hetero structure (for a microoptical resonator) which employs as the luminescent layer a thin film of tris(theonyltrifluoroacetonato) (4,7-dipheyl-1,10-phenanthroline)-Eu(III) complex was reported by N. Takada, T. Tsutsui, and S. Saito in Japanese Journal of Applied Physics, vol. 33, Part 2, No. 6B, pp. L863-L866 (1994).
An organic EL element that emits red light by means of a tris(theonyltrifluoroacetonato)(1,10-phenanthroline)-Eu(III) complex contained in the electron transport layer was reported by T. Sano, M. Fujita, T. Fujii, and Y. Hamada in Japanese Journal of Applied Physics, vol. 34, Part 1, No. 4A, pp. 1883-1887.
As mentioned above, it is possible to produce red light by means of a variety of luminescent materials; however, there still remain problems to be solved about color purity, luminance, and stability. A new luminescent material for red light is to be developed.
An organic EL element of single-hetero structure (as shown in FIG. 4) was reported by C. W. Tang and S. A. VanSlyke in Applied Physics Letters, vol. 51, No. 12, pp. 913-915 (1987). This EL element is of two-layer structure, composed of an organic thin film of hole transporting material and a thin film of electron transporting material, so that it emits light upon recombination of holes and electrons injected into the organic thin film from respective electrodes.
The double-layer structure contributes to a great reduction in driving voltage and improvement in luminous efficiency, because either the hole transporting material or the electron transporting material functions also as the luminescent material and light emission takes place in the wavelength band corresponding to the energy gap between the ground state and the excited state of the luminescent material.
Later, an EL element of double-hetero structure (which is composed of three layers of hole-transporting material, luminescent material, and electron-transporting material) was reported by C. Adachi, S. Tokito, and S. Saito in Japanese Journal of Applied Physics, vol. 27, No. 2, pp. L269-L271 (1988). In addition, an EL element constructed such that the electron transporting material contains the luminescent material, was reported by C. W. Tang, S. A. VanSlyke, and C. H. Chen in Journal of Applied Physics, vol. 65, No. 9, pp. 3610-3616 (1989).
These studies proved the feasibility of an EL element which emits high-luminance light at a low voltage. Active researches and developments in such an EL element are under way.
As a matter of fact, there are many problems for solution before it is put to practical use. Above all, what is important is the development of a luminescent material which emits red light with a high color purity and a stable high luminance. Known examples of the red luminescent material include red fluorescent organic dyes and europiumametal complexes; however, they are not satisfactory.
As for the fluorescent organic dye, an EL element that employs 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran contained in the electron transporting material, was reported by C. W. Tang, S. A. VanSlyke, and C. H. Chen in Applied Physics Letters, vol. 51, No. 12, pp. 913-915 (1987).
Also, an organic EL element (for white light) that employs Nile Red (as the red luminescent material) contained in the electron transporting material, was reported by J. Kido, M. Miura, and K. Nagai in Science vol. 267, pp. 1332-1334 (1995).